The embodiment relates to a DC-DC converter and a method of operating the same. More particularly, the embodiment relates to a DC-DC converter, capable of improving switching performance and switching efficiency with the reduced volume and the reduced cost, and a method of operating the same.
As industries have been developed and new generation energy has been extensively supplied, studies and research on high-capacity DC-DC converters (DC power converting devices) have been actively carried out. The high-capacity DC-DC converters have been variously applied to a power supply, a low-voltage/high-capacity conversion device, or a high-voltage conversion device in the fields such as broadcasting communication appliances, OA appliances, industrial electronic appliances, micro-computers, and ozone apparatuses.
In addition, generally, pollution-free vehicles using electricity employ DC-DC converters in order to greatly improve the efficiency of systems, and an insulating function is necessarily required for the safety. In particular, the required power capacity of the electronic units is increased with the advance of a battery technology and the increase of power required in a vehicle. The input/output voltage of a DC-DC power device such as an OBC (on-board charger) or an LDC (low voltage DC-DC converter) is set to a specific voltage (grid voltage or battery voltage). Accordingly, the increase of the power capacity may refer to the increase of input/output current. Since the performance of the charging electronic units directly exert an influence on electricity charges or the driving distance of a vehicle, the high-performance DC-DC power device having a high capacity-high current characteristic has been required.
The DC-DC converter employs various converters such that the DC-DC converter is highly integrated and represents high efficiency while operating in a wide input/output voltage and current range. In particular, a zero-voltage or zero-current switching scheme has been used to reduce switching loss and switching stress by switching a switch at zero-voltage or zero current.
FIG. 1 is a circuit diagram showing a phase-shifted full-bridge (PSFB) converter.
Referring to FIG. 1, a phase-shifted full-bridge converter may include an input unit 11 connected with a battery or an AC-DC PFC output terminal, a switch unit 12 to convert DC input to AC input, a transformer 13 to perform transformation based on an insulating ratio and the ratio of transformation, a rectifying unit 14 to convert AC voltage into DC voltage, a filter unit 15 to smooth voltage, and an output unit 16 for connection of the battery.
Hereinafter, the operation of a typical phase-shifted full-bridge converter of FIG. 1 will be described with reference to FIG. 2.
During a half-period, the PSFB converter applies voltage to a transformer by conducting an upper switch S1 having a leading leg and a lower switch S4 having a lagging leg. The PSFB converter adjusts a voltage ratio by adjusting the phase for overlapping period of the switches S1 and S4 differently from a conventional full-bridge converter to convert the voltage ratio by adjusting a duty ratio. That is to say, in the PSFB converter, the soft switching of each switch can be implemented by using phase adjustment, which representing improved efficiency as compared with a conventional scheme.
The PSFB circuit has been implemented in most products in the high capacity DC-DC converter. The RSPB allows the soft switching of all switches by using a simple control scheme (phase angle control scheme). However, the soft switching of the lagging-leg switch cannot be implemented in a light load.
In addition, although the soft switching can be implemented by using the output current in the case of the leading-leg switch, zero-voltage switching must be performed by using only circulating current at a primary side. Accordingly, resonance energy is insufficient in the light load.
Further, in the case of a wide input voltage range, energy shortage is represented in high-voltage input, so that efficiency can be degraded. In order to prevent the above phenomenon, an inductor may be inserted between a switch unit and a transformer unit. However, the transition time of secondary current is reduced, and the effective conductive ratio is reduced.
Meanwhile, although the inductor of the filter unit is required in order to reduce current ripples, the conduction loss is increased by winding in the case of high-current output. Since a large core must be used in order to prevent magnetic saturation, the increase of the volume and the weight is caused.